How can I create a silhouette using HDR in cycles?
A silhouette means that the light coming from the camera side of your object is not as intense as that of the background by a large ratio. Bringing down the intensity of the image used in the environment can help you bring down values on the foreground object as well.

There are several ways to adjust the exposure for a scene

Use the strength slider to adjust the exposure on the HDR, this will allow you to adjust the HDR environment without affecting any of the intensity of other lights on the scene:



A different way to do this is to Change the exposure for the whole scene on the Film>Exposure section. Changing the exposure will affect all of the elements in the scene:



Or you can also change the exposure value on the color management section:



If you use the compositor (which is not only not a hack, but it is your very best friend, as it can be a powerful tool) you can use one of the many color correction tools available in it, in this case the ASC-CDL node is the best tool for the job.

The main advantage of this approach is that you can alter the relationship between the bright and dark information of the scene. Not only you can change the exposure making all of the image brighter or darker, but if you you modify the power you will affect the ratios (or contrast) of the scene, giving you much more nuanced control on the final result. 



You can even decide to keep a bit more detail in the highlights and the shadows: